1. Field of the Invention
The present invention relates to microorganisms and methods for producing L-threonine. In particular, the present invention relates to the production of L-threonine using microorganisms, and Escherichia coli strains in particular, which require L-methionine, are L-isoleucine-leaky for growth, are resistant to α-methylserine, diaminosuccinic acid, L-glutamic acid, L-threonine, medium containing 60% of L-threonine fermentation mother liquid (which contains more than 9.0% threonine), azetidine and dehydroproline, and which are susceptible to fluoropyruvate.
2. Description of the Related Art
L-threonine, an essential amino acid, is a second limited amino acid of rice. As is well known, 1-threonine is used as a component for, e.g., amino acid transfusion liquid or general amino acid tablets, and as a nutrient. Recently, there has been a great increase in the demand for L-threonine because it, together with L-lysine, is used as an additive in feedstuff.
Japanese Pat. Publication No. Heisei 5-10076 teaches use of recombinant DNA which contains the genetic information for asparto kinase, homoserine kinase, homoserine dehydrogenase, and threonine synthase in production of threonine from a L-threonine-producing Serratia sp. Japanese Pat. Publication No. Heisei 1-289493 discloses that a DNA taken from a Providencia sp. resistant to methionine metabolic antagonist is genetically engineered and used to increase the productivity of L-threonine. In order to produce L-threonine, a threonine metabolic antagonist-resistant Escherichia sp. which requires methionine or diaminopimelic acid for growth has been used (Japanese Pat. Publication No. Sho. 56-10037). A strain which can grow in medium containing L-serine and ethionine has also been used (EP 91103569.9).